1. Field of the Invention
The present invention relates to a pressure-reaction synthesis (PRS) process for forming titanium metal matrix composites by controlled chemical reaction of a mixture of metal and nonmetal powders under specific conditions of temperature within a forming die under specific conditions of pressure.
2. Background of the Invention
Titanium and its alloys are used in the construction of parts for machines and equipment because of their high strength and toughness, their excellent corrosion resistance and their low density; however, it is in the interest of the government and the citizens of the United States to provide titanium composite materials capable of providing the best available protection to personnel and equipment in all conditions where a threat to safety exists.
The manner of providing the best available protection to personnel and equipment would be to enhance protection per unit of thickness over existing conventional armor at a significant weight savings. Moreover, lighter personnel armor improves troop mobility and comfort, and weight reduction owing to lighter armor can result in a gross vehicle weight reduction of 2-10 times the actual weight decrease of the armor. Such a weight reduction significantly improves the operational economy of vehicles and increases the unit carrying capacity of transport vehicles, ships and aircraft.
In general, in the case of titanium or titanium alloy materials, the prior art teaches powder metallurgy technology for preparing metal matrix composites MMCs and laminated structures through the use of fine powders of an inert phase or phases (TiC, TiN, TiB and TiB.sub.2) dispersed in Ti or Ti alloy powders. These powders are thoroughly mixed and consolidated into a green body by cold-compaction within a rigid die or by cold isostatic pressing within a flexible elastomer. The green body is densified by sintering and canning followed by hot isostatic pressing. Densification of the green body may also be accomplished by sintering to a state of non-connected porosity followed by hot isostatic pressing as is demonstrated in U.S. Pat. No. 4,731,115.
While powder metallurgy is a mature technology which often provides complicated shapes, efficient use of materials (near-net-shapes) and products with unique properties, one known limitation of the prior art is the lengthy time period at given temperatures that are necessary for densification by diffusion during sintering or hot isostatic pressing. For example, the required time and temperature are in the order of from about 1 to about 4 hours at from about 1150.degree. C. to 1400.degree. C. When the time at temperature is extended, composition leveling occurs, which is contamination of the matrix by diffusional interaction with the dispersed phase or phases. In the case of titanium or titanium alloy substrates coated with mixtures of metal, hard-metal or ceramic powders, which on sintering become metal matrix composites (MMCs), or laminated composites comprised of alternating layers of metallic substrates and MMCs, extended sintering results in weak bonding owing to the formation of brittle phases at the substrate interface and/or by contamination of the interface or both.
Further, extended sintering at high temperatures, as required in conventional powder metallurgical technology often results in recrystallization of the metal substrate accompanied by attendant degradation of mechanical properties.
Another limitation of the prior art in this area is the limited variety of dispersed phases that can be used in titanium metal matrix composites. As an example, the prior art is only applicable to dispersed phases that do not interact by diffusion or chemical reaction with titanium. A still further limitation of the prior art is the high pressures required for densification, and since pressure tends to improve densification, the prior art often operates at the pressure limit of the apparatus being used.
Accordingly, there is a need in powder metallurgy technology used for preparing metal matrix composites in laminated structures through the use of fine powders to develop processes requiring shorter times, lower temperatures and lower pressures in order to obtain titanium composite materials having properties heretofore unattainable using prior art techniques.
A further need exists in the art of preparing titanium composite materials to provide a pressure-reaction synthesis process which requires short reaction times of between about 5 to about 30 minutes in order to avoid excessive diffusional interaction (diffusional leveling) between components within a MMC or PRS bonding layer.
A yet further need exists in the art of preparing titanium composite materials to provide shorter reaction times to avoid excessive diffusional interaction between components within a MMC or PRS bonding layer, thereby increasing the fraction of dispersed phase within a composite without contaminating and embrittling the matrix material.